Solid electrolytic capacitor

ABSTRACT

A solid electrolytic capacitor comprises a capacitor element including a chip body and an anode wire projecting from the chip body, an anode lead electrically connected to the anode wire, a cathode lead paired with the anode lead, a fuse wire having a first end electrically connected to the cathode lead and a second end electrically connected to the chip body, and a resin package enclosing the capacitor element, part of the anode lead, part of the cathode lead, and the fuse wire. The first end of the fuse wire has a nail head form for bonding to the cathode lead with a sufficient adhesion area without increasing the length of the first end.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to solid electrolytic capacitors such as solidtantalum or aluminum capacitors. More specifically, the presentinvention relates to a solid electrolytic capacitor of the type whichhas a built-in fuse for improving the safety of the capacitor.

2. Description of the Prior Art

A solid electrolytic capacitor having a built-in safety fuse isdisclosed for example in "NEC Technical Report" Vol. 44, No. 10/1991,Pages 116-120 or Japanese Patent Application Laid-open No. 2-105513.Such a capacitor is also illustrated in FIGS. 12-14 of the accompanyingdrawings.

As shown in FIGS. 12-14, the prior art solid electrolytic capacitorcomprises a capacitor element 101 which includes a chip body 101a(sintered mass of metal particles) and an anode wire 101b projectingfrom the chip body. The capacitor further comprises an anode lead 102electrically connected to the anode wire 101b by welding for example,and a cathode lead 103 electrically connected to the chip body 101athrough a fuse wire 104. The fuse wire 104 may be made to break uponoverheat in the case of a temperature fuse or upon passage of anovercurrent in the case of an overcurrent fuse.

The fuse wire 104 is partially enclosed in a relatively softarc-extinguishing member 106 which may be made of silicone resin. Thecapacitor element 101 together with the fuse wire 104 and part of therespective leads 102, 103 is enclosed in a protective package 105 whichis made of a relatively hard resin such as epoxy. The projectingportions of the respective leads 102, 103 are bent to engage theunderside of the package 105.

In such a prior art capacitor, one end (first end) 104a of the fuse wire104 is connected to the cathode lead 103 by causing a bonding tool 107to press the first end 104a, as best shown in FIG. 14. As a result, thefirst end 104a is flattened for bonding to the cathode lead 103 with asufficient adhesion area.

However, the above manner of bonding is disadvantageous in the followingrespects.

(1) Since the first end 104a of the fuse wire 104 is cross-sectionallyreduced due to flattening, the fuse wire 104 tends to become extremelyweak near the flattened first end 104a. Thus, at the time of molding theresin package 105, the fuse wire 104 is likely to break near theflattened first end 104a under the influences of the resin injectionpressure.

(2) The breaking temperature and/or current of the fuse wire 104 aredetermined by the cross-sectionally smallest portion of the fuse wirewhich is located near the flattened first end 104a. Since the degree offlattening cannot be strictly equalized with respect to different fusewires, it is inevitable that the breaking characteristics of differentproducts varies.

(3) The flattened first end 104a of the fuse wire 104 must have asufficient length H (see FIG. 12) for providing a sufficient bondingstrength. Thus, the bonding portion of the cathode lead 103 must becorrespondingly elongated to result in an increase of the length L1 ofthe capacitor, thereby hindering a reduction in the size and weight ofthe product.

On the other hand, it is also possible to connect the first end 104a ofthe fuse wire 104 to the cathode lead 103 by soldering without invitingthe problems (1) and (2) described above. However, the solderingoperation is relatively time-taking, and the necessity of separatelyusing solder adds to the production cost. Further, the problem (3) abovecannot be fully solved by the use of solder.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a solidelectrolytic capacitor, particularly a tantulum capacitor, wherein afuse wire can be electrically connected to a cathode terminal with asufficient mechanical strength without increasing the bonding length ofthe fuse wire and without cross-sectionally constricting the fuse wirenear its bonding end.

Another object of the invention is to provide a solid electrolyticcapacitor wherein the fuse wire can be electrically connected to acapacitor chip body without damaging it.

According to the present invention, there is provided a solidelectrolytic capacitor comprising: at least one capacitor elementincluding a chip body and an anode wire projecting from the chip body;an anode lead electrically connected to the anode wire; a cathode leadpaired with the anode lead; a fuse wire having a first end electricallyconnected to the cathode lead and a second end electrically connected tothe chip body; and a resin package enclosing the capacitor element, partof the anode lead, part of the cathode lead, and the fuse wire; whereinthe first end of the fuse wire has a nail head form for bonding to thecathode lead.

Preferably, a portion of the fuse wire immediately following the nailhead first end may be made to extend generally perpendicular to thecathode lead, thereby maximizing the size reduction.

According to a preferred embodiment, the second end of the fuse wire isflattened, particularly into a generally discal form, for bonding to thechip body without cross-sectionally constricting the fuse wire near theflattened second end. In this case, the discal second end of the fusewire may be obtained by flattening a ball end of the fuse wire at thetime of bonding to the chip body. Alternatively, the discal second endof the fuse wire may be obtained by flattening a ball end of the fusewire before bonding to the chip body.

Other objects, features and advantages of the present invention willbecome apparent from the following detailed description of the preferredembodiments given with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a view, in vertical section, showing a solid electrolyticcapacitor according to the present invention;

FIG. 2 is a sectional view of the same capacitor taken along linesII--II;

FIG. 3 is a perspective view of the same capacitor;

FIG. 4 is a perspective view showing a method of manufacturing the samecapacitor;

FIG. 5 is a view, in vertical section, showing another solidelectrolytic capacitor according to the present invention;

FIG. 6 is a sectional view taken along lines VI--VI in FIG. 5;

FIGS. 7 and 8 are views showing a method of making the capacitor shownin FIG. 5;

FIGS. 9 and 10 are views showing another method of making the capacitorshown in FIG. 5;

FIG. 11 is a perspective view showing a collective-type solidelectrolytic capacitor according to the present invention;

FIG. 12 is a view, in vertical section, showing a prior art solidelectrolytic capacitor;

FIG. 13 is a sectional view taken along lines XIII--XIII in FIG. 12; and

FIG. 14 is a perspective view of the same prior art capacitor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 to 3 of the accompanying drawings show a solid electrolyticcapacitor according to a first embodiment of the present invention. Thecapacitor comprises a capacitor element 1 which includes a chip body 1a(sintered mass of metal particles) and an anode wire 1b projecting fromthe chip body. The capacitor further comprises an anode lead 2electrically connected to the anode wire lb by welding for example, anda cathode lead 3 electrically connected to the chip body 1a through afuse wire 4. The fuse wire 4 is partially enclosed in a relatively softarc-extinguishing member 6 which may be made of silicone resin.

The capacitor element 1 together with the fuse wire 4 and part of therespective leads 2, 3 is enclosed in a protective package 5 which ismade of a relatively hard resin such as epoxy. The projecting portionsof the respective leads 2, 3 are bent to engage the underside of thepackage.

In the first embodiment, the fuse wire 4 is made of solder wherein theproportion of lead (Pb) and tin (Sn) is selected to have a melting pointof about 300° C. This melting point is selected so that the fuse wire 4will break (by melting) at a dangerously high temperature of above 300°C. while preventing breakage under the heat generated at the time ofsoldering the capacitor to a suitable portion of a printed circuit board(not shown) for die-bonding.

The diameter of the fuse wire 4 may be selected in the range of about50-120 micrometers depending on the required breaking characteristics.For example, if the fuse wire 4 has a diameter of 80 micrometers, itwill break upon passage of 1-2A current for about 10 seconds. On theother hand, if the fuse wire 4 has a diameter of 120 micrometers, itwill break upon passage of 5A current for about 5 seconds.

According to the first embodiment, one end (first end) 4a of the solderfuse wire 4 is electrically connected to the cathode lead 3 by firstforming a ball at the first end 4a and thereafter pressing the ball endagainst the cathode lead 3 generally perpendicularly thereto butlongitudinally of the fuse wire 4 under the application of ultrasonicvibration and/or heat. After connection to the cathode lead 3, the firstend 4a of the wire 4 is deformed to have a nail head form.

The other end (second end) 4b of the solder fuse wire 4 may beelectrically connected to the chip body 1a by pressing the second end 4bagainst the chip body 1a under application of ultrasonic vibrationand/or heat. Alternatively, the second end 4b of the wire 4 may beconnected to the chip body 1a by soldering or by using an electricallyconductive paste.

The solid capacitor having the above-described configuration may bemanufactured in the following manner.

As shown in FIG. 4, use is made of a leadframe 10 which integrallyincludes a plurality of anode leads 2 and a plurality of cathode leads 3paired with the respective anode leads. During longitudinal transfer ofthe leadframe 10 (indicated by an arrow in FIG. 4), a plurality ofcapacitor elements 1 are mounted to the leadframe by connecting theirrespective anode wires 1b to the respective anode leads 2.

Then, a vertically movable capillary tool 11 for supplying a solder fusewire 4 is arranged immediately above a selected cathode lead 3, as shownat the right-hand position in FIG. 4. The lower end 4a of the fuse wire4 is made to have a ball formed by thermal melting, and the ball end 4ais pressed against the cathode lead 3 under the application ofultrasonic vibration and/or heat for connection thereto by lowering thecapillary tool 11.

Then, the capillary tool 11 is raised while allowing the fuse wire 4 tobe paid out. When the capillary tool is raised by a predeterminedamount, the fuse wire 4 is cut at a suitable position of the wire toprovide an non-connected upper end 4b, and a new ball (not shown) isthermally formed at the lower end of the fuse wire 4 still remaining onthe capillary tool 11.

Then, the upper end 4b of the fuse wire 4 is bent toward the chip body1a of the capacitor element 1 by advancing a bending tool 12 which ismovable generally horizontally back and forth.

Then, the upper end 4b of the fuse wire 4 is pressed against the chipbody 1a of the capacitor element 1 under the application of ultrasonicvibration and/or heat by lowering a vertically movable bonding tool 13,as shown at the left-hand in FIG. 4. As a result, the upper end 4b ofthe fuse wire is electrically bonded to the capacitor chip body 1a.

Finally, the arc-extinguishing member 6 (FIG. 1) and the resin package 5(also FIG. 1) are formed, and the product is obtained by cutting therespective leads 2, 3 off the leadframe 10.

The solid electrolytic capacitor described above has the followingadvantages.

First, since the first end 4a of the fuse wire 4 has a nail head form toincrease the adhesion area relative to the cathode lead 3 withoutincreasing the length of the first end 4a itself, it is possible todecrease the length of the cathode lead 3, thereby enabling to reducethe length L (FIG. 1) of the capacitor as a whole. Obviously, such asize reduction also contributes to a reduction in the weight and cost ofthe capacitor.

Secondly, due to the formation of the nail head end 4a, the fuse wire 4has no cross-sectionally reduced portion which would be easily broken atthe time of molding the resin package 5 and which would result invariations of the breaking characteristics. Thus, it is possible toincrease the yield of production and equalize the breakingcharacteristics from product to product.

In the third place, since the fuse wire 4 is made to extendperpendicularly to the cathode lead 3 at a position adjacent to the nailhead end 4a, the cathode lead 3 may be located as close to the chip body1a as possible. Such an arrangement also contributes to a reduction ofthe length L of the capacitor.

FIGS. 5 and 6 show a solid electrolytic capacitor according to a secondembodiment of the present invention. The capacitor of the secondembodiment is similar to that of the first embodiment but differstherefrom only in that the fuse wire 4 has an enlarged, generally discalend (second end) 4b' for electrical connection to the capacitor chipbody 1a in addition to the enlarged nail head end (first end) 4a forelectrical connection to the cathode lead 3.

The bonding of the fuse wire 4 may be preferably performed in thefollowing manner.

As shown in FIGS. 7 and 8, a combination of a heater block 14 and acover plate 15 is used for bonding the fuse wire 4 to a selected cathodelead 3 of a leadframe 10 and to a corresponding capacitor chip body 1a.The heater block 14 has a tunnel 16 in which the capacitor element 1fixed to the leadframe 10 is arranged, whereas the cover plate 15 has anopening 17 communicating with the tunnel 16. Above the opening 17, thereare arranged a capillary tool 11 for continuously supplying a solderfuse wire 4, a bending tool 12, a bonding tool 13, and a torch 18.

During the bonding operation, a reducing gas (containing nitrogen gasmixed with about 4-5% of hydrogen gas for example) or an inert gas(containing only nitrogen gas for example) is supplied to the tunnel 16of the heater block 14 from below for discharging through the opening 17of the cover plate 15, so that a reducing or inert atmosphere is createdimmediately above the opening 17. In this condition, the lower end 4a ofthe fuse wire 4, which has been previously melted into a ball, is firstbonded to the cathode lead 3 by lowering the capillary tool 11 forpressing the ball end 4a to the cathode lead 3 under application of heat(provided by the heater block 14) and/or ultrasonic vibration.

Then, the capillary tool 11 is raised while allowing the fuse wire 4 tobe paid out. When the capillary tool is raised by a predeterminedamount, the torch 18 is brought closer to the fuse wire 4 for thermalcutting thereof. As a result, the shorter portion of the fuse wire 4connected to the cathode lead 3 is made to have an upper ball end 4b',whereas the other portion of the fuse wire 4 still remaining on thecapillary tool 11 is made to have a lower new ball end 4a. Obviously,the new ball end 4a is used for bonding to the next cathode lead (notshown).

Then, the upper ball end 4b' of the shorter fuse wire 4 is bent towardthe chip body la of the capacitor element 1 by advancing the bendingtool 12 generally horizontally, as indicated by phantom lines in FIG. 7.

Then, the upper ball end 4b' of the shorter fuse wire 4 is pressedagainst the chip body la of the capacitor element 1 under application ofheat and/or ultrasonic vibration by the vertically movable bonding tool13. As a result, the upper ball end 4b' of the shorter fuse wire 4 isflattened into a generally discal form for electrically bonding to thecapacitor chip body 1a (see FIG. 6).

Instead of immediately pressing against the cathode lead 3, the upperball end 4b' of the fuse wire 4 may be first flattened into a discalform by compressing between a pair of clamp members 19a, 19b, as shownin FIG. 9. Thereafter, the flattened discal end 4b' is brought towardthe capacitor chip body 1a by advancing the bending tool 12 (FIG. 7) andpressed against the chip body la for bonding thereto by lowering thebonding tool 13 (also FIG. 7), as shown in FIG. 10.

In either of the two bonding methods illustrated in FIGS. 7 to 10, thediscal end 4b' of the fuse wire 4 for bonding to the capacitor chip body1a is formed by flattening the upper ball end of the wire. Obviously,the discal wire end 4b' provides an increased adhesion area as requiredfor insuring a sufficient bonding strength. In addition, it isunnecessary to separately use solder or conductive paste for electricalconnection, thereby facilitating the bonding operation and reducing theproduction cost.

Further, despite flattening for increasing the adhesion area, thethickness D1 (see FIG. 10) of the discal wire end 4b' can be renderedgenerally equal to the diameter of the fuse wire 4 due to theutilization of the ball end. Thus, near the discal end 4b', the fusewire 4 will have no cross-sectionally reduced portion which would beeasily broken at the time of molding the resin package 5 (FIG. 5) andwhich would result in variations of the breaking characteristics.

In the bonding method illustrated in FIGS. 9 and 10, the upper ball end4b' of the fuse wire 4 is flattened before bonding to the capacitor chipbody 1a. Thus, the subsequent bonding of the thus flattened discal end4b' of the fuse wire 4' relative to the capacitor chip body 1a can beperformed with a smaller bonding force than required for flattening thewire ball end simultaneously with bonding to the chip body. As a result,the chip body 1a, which is a sintered mass of metal particles, is lesslikely to be damaged (e.g. material chipping or crack formation) at thetime of bonding the fuse wire 4.

The present invention may be applied to a collective-type solidelectrolytic capacitor, as shown in FIG. 11. More specifically, thecollective-type capacitor includes a plurality of capacitor elements 1'arranged in parallel to each and enclosed in a common resin package 5'.The respective capacitor elements 1' may differ from each other incapacitance. Alternatively, all of the capacitor elements may have anequal capacitance.

The collective-type capacitor of FIG. 11 is shown to include separateanode leads 2' for the respective capacitor elements 1' but a commoncathode lead 3' for electrical connection to the respective capacitorelements through separate fuse wires 4'. Alternatively, use may be madeof a common anode lead for all of the capacitor elements 1' and separatecathode leads for the respective capacitor elements.

The present invention being thus described, it is obvious that the samemay be varied in many ways. For instance, the fuse wire 4 (or 4') may bemade of gold, copper or aluminum in place of solder. Such variations arenot to be regarded as a departure from the spirit and scope of theinvention, and all such modifications as would be obvious to thoseskilled in the art are intended to be included within the scope of thefollowing claims.

I claim:
 1. A solid electrolytic capacitor comprising:at least onecapacitor element including a chip body and an anode wire projectingfrom the chip body; an anode lead electrically connected to the anodewire; a cathode lead paired with the anode lead; a fuse wire having afirst end electrically connected to the cathode lead and a second endelectrically connected to the chip body, the fuse wire being made ofsolder; and a resin package enclosing the capacitor element, part of theanode lead, part of the cathode lead, and the fuse wire; wherein thefirst end of the fuse wire has a nail head form for bonding to thecathode lead.
 2. The capacitor according to claim 1, wherein a portionof the fuse wire immediately following the nail head first end extendsgenerally perpendicularly to the cathode lead.
 3. The capacitoraccording to claim 1, wherein the second end of the fuse wire isflattened for bonding to the chip body without cross-sectionallyconstricting the fuse wire near the flattened second end.
 4. Thecapacitor according to claim 3, wherein the flattened second end of thefuse wire has a generally discal form.
 5. The capacitor according toclaim 4, wherein the discal second end of the fuse wire has a thicknesswhich is generally equal to a diameter to the fuse wire itself.
 6. Thecapacitor according to claim 4, wherein the discal second end of thefuse wire is obtained by flattening a ball end of the fuse wire at thetime of bonding to the chip body.
 7. The capacitor according to claim 4,wherein the discal second end of the fuse wire is obtained by flatteninga ball end of the fuse wire before bonding to the chip body.
 8. Thecapacitor according to claim 1, wherein the resin package encloses atleast one additional capacitor element.
 9. The capacitor according toclaim 8, wherein one of the anode and cathode leads is provided commonlyfor the first-mentioned and additional capacitor elements.